1. Field of the Invention
The present invention relates to a projection exposure apparatus, and particularly, to an exposure apparatus in which adverse influences of aberrations of a projection lens such as symmetrical distortion, curvature of field and the like of the projection lens to a high resolution are reduced or eliminated.
2. Related Background Art
In recent years, a projection type exposure apparatus has been widely used as an apparatus for producing a semiconductor integrated circuit in order to obtain a higher resolution, a higher exposure accuracy and a greater throughput.
In the projection exposure apparatus, there is provided a projection lens between a reticle (an original plate) and a silicon wafer (a substrate), and a circuit pattern formed on the reticle is imaged on the silicon wafer with a predetermined reduction ratio or reduced magnification (normally 1/5 or 1/10) through the projection lens. Thus, a pattern image which is a reduced image of the circuit pattern is formed on the silicon wafer. In a reduction projection type exposure apparatus, a step-and-repeat exposure system is generally adopted in which a silicon wafer is shifted each time one exposure has been completed and plural pattern images are arranged over an entire silicon wafer without any clearance by plural exposures, rather than a one-shot exposure system in which a pattern image is formed over an entire silicon wafer by a single exposure.
However, in order to cope with yet a further higher integration of a semiconductor integrated circuit, a limit to resolution which occurs in only using a reduction projection exposure apparatus to project a circuit pattern at a reduced ratio should be solved. So, it is required that the wavelength of an exposure light be made shorter and that an NA of a projection lens be made larger to improve the resolution. In this case, however, the following problems in turn arise due to a reduction of depth of focus resulting from the enlarged NA of a projection lens and due to a greater accuracy in alignment between a reticle and a silicon wafer resulting from a refined circuit pattern.
(1) Shift of a focal point of a projection lens:
The shift occurs because the projection lens absorbs part of the exposure light, and the displacement of the focal point increases as the exposure proceeds, as shown in FIG. 1. When the exposure starts at a time t.sub.0, the displacement increases with time and a stationary state is reached at a time .sub.1. Then, if the exposure terminates at a time t.sub.2, the displacement decreases with time and becomes zero at a time t.sub.3. The displacement of the focal point would not be any trouble so long as its amount is small enough. But, if the displacement surpasses the range of a depth of locus, it becomes a great problem.
(2) Shift of an image magnification of a projection lens:
The shift also occurs since the projection lens absorbs part of the exposure light. As shown in FIG. 2, the displacement of the image magnification increases as the exposure proceeds. When the exposure starts at a time t.sub.0, the displacement increases with time and a stationary state is reached at a time t.sub.1. Then, if the exposure terminates at a time t.sub.2, the displacement decreases with time and becomes zero at time t.sub.3. Displacement of the image magnification would directly influence an arranging accuracy of circuit patterns on the wafer, and this becomes a great problem when a refined circuit pattern is depicted on the wafer.
(3) Symmetrical distortion of a projection lens:
The distortion also occurs since the projection lens absorbs part of the exposure light. The distortion could not be removed even if a projection lens of high performance is used which is designed to have no distortion at a stage of production thereof.
(4) Curvature of field of a projection lens:
Curvature of the field also occurs since the projection lens absorbs part of exposure light. When a wafer whose exposure area is enlarged to 20 mm.times.20 mm, it is difficult to maintain a best focused state over an entire exposure area of the wafer due to the shift of curvature of field of the projection lens. Thus, a limit to higher resolution occurs.
In a conventional projection exposure apparatus, the problems of (1) and (2) discussed above are solved by the following measures (a) and (b), and even if a focal point and an image magnification of the projection lens are greatly varied, these variations are corrected so that a circuit pattern on a reticle can be transferred on a silicon wafer under permissible conditions. But, the above-noted problems (3) and (4) are not yet be solved.
(a) Measures to correct a shift of a focal point:
A stage on which a silicon wafer is mounted is moved in a direction of an optical axis of the projection lens. Thus, a distance between the silicon wafer and the projection lens is adjusted according to the displacement of the focal point of the projection lens.
(b) Measures to correct a shift of an image magnification:
A sealed space is provided in a given space between lenses consisting of a projection lens. The air pressure in the sealed space is adjusted according to the shift of the image magnification of the projection lens (see Japanese Patent Laid-Open No. 60-078454). Or, a distance between the reticle and the projection lens, or distances between respective lenses consisting of the projection lens is adjusted according to the shift of the image magnification.
Therefore, in order to obtain a still further higher resolution in the conventional projection exposure apparatus, the problem concerning the symmetrical distortion of the projection lens as shown in FIGS. 3A and 3B or the curvature of field thereof must be solved.
In more detail, in a reduced projection exposure apparatus wherein only the correction of a shift of a focal point of a projection lens and the correction of an image magnification are performed, there is a problem that a pattern image 201 formed on a wafer as shown by a solid line in FIG. 3A has a barrel type distortion in which four sides of a square are curved outwardly, compared with a regular pattern image 200 as shown by dotted lines, when the projection lens involves a barrel type distortion which is a kind of symmetrical distortion. On the other hand, there is a problem that a pattern image 202 formed on a wafer as shown by a solid line in FIG. 3B has a reel type distortion in which four sides of a square are curved inwardly, compared with a regular pattern image 200 as shown by dotted lines, when the projection lens involves a reel type distortion which is also a kind of symmetrical distortion.
When the projection lens involves a curvature of field which is a kind of aberration, the pattern image partially shifts in a direction perpendicular to an exposure surface from a regular pattern image.